The present invention pertains generally to the field of sensors for the measurement of changes in mass and heat flow. More particularly, the present invention pertains to multiple microresonator mass and heat flow sensors which may provide simultaneous and continuous measurement of the changes in mass and heat flow at a gas-solid interface. The present invention also pertains to an apparatus comprising such sensors as sample and reference sensors, and to methods of measuring the mass and heat flow of a gas, liquid, or solid sample by utilizing such apparatus.
Although the piezoelectric effect has been known since the 19th century, the development of quartz crystal devices which oscillate at precisely defined resonant frequencies and which can be incorporated as passive elements into electronic instruments began in the 1920""s. Like much of our modem electronic technology, their development received a massive push during World War II, when over 30 million quartz crystal oscillators were produced for use in military communications equipment. Today there is widespread use of quartz crystal oscillators and of new types of microresonators in electronics wherever precise control of frequency is needed as, for example, in radio frequency communications, in frequency meters and timepieces, in scientific instrumentation, and in computers and cellular telephones.
There are several useful books which describe the physics of quartz crystal oscillators and other microresonators and their use in electronic circuits. For example, Introduction to Quartz Crystal Unit Design by Bottom, Van Nostrand Reinhold, New York, 1982, discusses the physical crystallography of quartz, mechanic vibrations and stress/strain relationships, the piezoelectric effect, the equivalent circuit of the quartz resonator and its use as a circuit component, the temperature stability of quartz oscillators, and other topics of importance in the application of these devices. Science, Vol. 249, pages 1000-1007 (1990), by Ward et al., describes the converse piezoelectric effect and its use in in-situ interfacial mass detection, such as in thickness monitors for thin-film preparation and in chemical sensors for trace gases. Analytical Chemistry, Vol. 65, pages 940A-948A and 987A-996A (1993), by Grate et al., compares the acoustical and electrical properties of five acoustic wave devices used as microsensors and transducers, including quartz crystal oscillators.
Any crystalline solid can undergo mechanical vibrations with minimum energy input at a series of resonant frequencies, determined by the shape and size of the crystal and by its elastic constants. In quartz, such vibrations can be induced by the application of a radio frequency voltage at the mechanical resonant frequency across electrodes attached to the crystal. This is termed the inverse piezoelectric effect. The thickness shear mode is the most common mechanical vibration used in quartz crystal oscillators. A typical commercially available quartz crystal oscillator is a thin circular quartz plate, cut from a single crystal at an angle of 37.25xc2x0 with respect to the crystal""s z axis (the so-called xe2x80x9cAT cutxe2x80x9d). This angle is chosen so that the temperature coefficient of the change in frequency is, to the first approximation, zero at 25xc2x0 C., thus minimizing the drift in resonant frequency with ambient temperature change. A slight change in the cut angle produces crystals with zero temperature coefficients at elevated temperatures. The AT-cut plate has thin film electrodes on most of the top and bottom surfaces of the crystal, and is supported in various ways at its circumference or perimeter. Both the fundamental and the first few overtones of the thickness shear mode have been utilized in crystal oscillators. A typical AT-cut quartz disk piezoid operating at a 10.8 MHz fundamental has the following dimensions, according to page 99 of the above-mentioned reference by Bottom:
The quality factor, Q, defined for any resonant circuit incorporating quartz crystal oscillators is usually not less than 105 and may be as high as 107. With careful attention to the control of temperature in a vacuum environment, a short-term frequency stability of one part in 1010 can be obtained, although the stated short-term stability for commercial units is xc2x13 ppm.
The resonant frequency of a quartz crystal oscillator is inversely proportional to the thickness, e, of the plate. For a circular disk,
f=nK/e
where n=1, 3, 5, . . . and K is the frequency constant (for example, see page 134ff of the above-mentioned reference by Bottom). For an AT-cut disk, K=1664 kHz mm, so that a disk of a thickness of 1 mm will oscillate at 1,664 MHz. If this thickness is increased by the deposition of material on the surface of the quartz crystal oscillator, then its frequency will decrease.
In 1957, Sauerbrey in Z. Physik, Vol. 155, 206 (1959), derived the fractional decrease in frequency f of a circular disk quartz crystal oscillator upon deposition of a mass, m, of material on its surface. The derivation relies on the assumption that a deposited foreign material exists entirely at the anti-node of the standing wave propagating across the thickness of the quartz crystal, so that the foreign deposit can be treated as an extension of the crystal, as, for example, described in Applications of Piezoelectric Quartz Crystal Microbalances by Lu et al., Elsevier, New York, 1984. Sauerbrey""s result for the fundamental vibrational mode is as follows:
xcex94ƒ/ƒ0=xe2x88x92xcex94e/e0=xe2x88x922ƒ0xcex94m/A{square root over (xcfx81xcexc)}
Here, xcex94e is the change in the original thickness e0, A is the piezoelectrically active area, xcfx81 is the density of quartz, and xcexc is the shear modulus of quartz. By measuring the decrease in frequency, one thus can determine the mass of material deposited on the crystal. This is the principle of the quartz crystal microbalance. In practice, the assumptions underlying the Sauerbrey equation are valid for deposits up to about 10% of the crystal mass, although the sensitivity to mass has been shown experimentally to decrease from the center of the electrode to its edge.
Torres et al. in J. Chem. Ed., Vol. 72, pages 67-70 (1995), describe the use of a quartz crystal microbalance to measure the mass effusing from Knudsen effusion cells at varying temperatures, in order to determine the enthalpies of sublimation. They reported a sensitivity of about 108 g/sec in the mass deposition rate. The application of the quartz crystal microbalance in chemistry for the sensitive detection of gases adsorbed on solid absorbing surfaces has been reviewed by Alder et al., in Analyst, Vol. 108, pages 1169-1189 (1983) and by McCallum in Analyst, Vol. 114, pages 1173-1189 (1989). The quartz crystal microbalance principle has been applied to the development of thickness monitors in the production of thin films by vacuum evaporation, as, for example, described in the above-mentioned reference by Lu et al. Quartz crystal oscillators of various sizes and modes of vibration are commonly used currently in research efforts in sensor development.
Throughout this application, various publications and patents are referred to by an identifying citation. The disclosures of the publications and patents referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
U.S. Pat. No. 5,339,051 to Koehler et al. describes resonator-oscillators for use as sensors in a variety of applications. U.S. Pat. No. 4,596,697 to Ballato and U.S. Pat. No. 5,151,110 to Bein et al. describe coated resonators for use as chemical sensors.
To overcome the influences of temperature changes on the microresonators, U.S. Pat. No. 4,561,286 to Sekler et al. and U.S. Pat. No. 5,476,002 to Bower et al. describe active temperature control or the use of temperature sensors with the microresonators. U.S. Pat. No. 5,686,779 to Vig describes a microresonator for direct use as a thermal sensor.
Microresonators, including quartz crystal microbalances (QCM""s), have been utilized to determine the mass changes with a variety of liquid samples such as, for example, described in U.S. Pat. No. 4,788,466 to Paul et al. When the microresonator is coated, chemicals present in the liquid samples may be detected as, for example, described in U.S. Pat. No. 5,306,644 to Myerholtz et al.
Microresonators have been adapted to measure the viscosity of a liquid sample as, for example, described in U.S. Pat. No. 4,741,200 to Hammerle. U.S. Pat. No. 5,201,215 to Granstaff et al. describes the use of microresonators to measure the mass of a solid and physical properties of a fluid in a sample.
Calorimeters for various types of heat measurements are well known as, for example, described in U.S. Pat. No. 4,492,480 to Wadso et al.; U.S. Pat. No. 5,295,745 to Cassettari et al.; and U.S. Pat. No. 5,312,587 to Templer et al. A combined scientific apparatus of a thermal analyzer, such as a calorimeter, and an X-ray diffractometer for observing simultaneously both thermodynamic and structural properties of materials is described in U.S. Pat. No. 4,821,303 to Fawcett et al.
Despite the various approaches proposed for the design of sensors and measurement systems based on microresonators as the sampling device, there remains a need for sensors and measurement systems which can simultaneously and continuously measure with high sensitivity and accuracy both mass and heat flow changes of a sample in contact with the microresonator.
One aspect of the present invention pertains to a new scientific apparatus based on the combination of two or more devices, wherein each device comprises (i) a microresonator mass sensor or microbalance, such as, for example, a quartz crystal microbalance (QCM), which may be used to measure very small changes of mass at its surface; and, (ii) a heat flow sensor, such as, for example, an isothermal heat conduction calorimeter (HCC), which may be used to measure small heat flows; and wherein at least one combined microresonator and heat flow sensor device is utilized as a reference device and at least one combined microresonator and heat flow sensor device is utilized as a sample device to measure the sample. In one embodiment, the dual microresonator and heat flow sensor sample and reference apparatus measures simultaneously and continuously, with high sensitivity (nanogram in mass, sub-microwatt in heat flow), the changes in mass and heat flow at a small gas-solid interface, for example, about 1 cm2 or less in area, due to chemical processes such as evaporation or condensation, adsorption or desorption, or gas-surface reactions. The new scientific apparatus of the present invention may be advantageously utilized in a variety of applications such as, for example, studying the hydration and dehydration of films of proteins and other biomolecules deposited on solid substrates, particularly for films utilized in biosensors, diagnostic immunoassays, the separation of proteins by chromatography, and as models for biological and biocompatible membranes and surfaces; studying the energetics of intermolecular interactions at the surface of polymer films and other organic surfaces important in adhesion, lubrication, wetting, and corrosion; and studying the energetics of the drying and curing of both water-based and organic solvent-based paints and finishes.
One aspect of the present invention pertains to a mass and heat flow measurement apparatus comprising (i) a sample sensor comprising a first microresonator, a first heat flow sensor, and a heat sink coupled thermally to the first heat flow sensor, wherein the first heat flow sensor is thermally coupled to the first microresonator; and further wherein the first microresonator is capable of measuring the mass of a sample in contact with the first microresonator, and the first heat flow sensor is capable of measuring the flow of heat from the sample to the heat sink; (ii) a reference sensor comprising a second microresonator, a second heat flow sensor coupled thermally to the second microresonator, and a heat sink coupled thermally to the second microresonator; and, (iii) a chamber housing the sample and reference sensors, wherein the reference sensor is isolated from the sample; and further wherein the second microresonator is capable of measuring a reference signal relating to mass at a surface of the second microresonator, and the second heat flow sensor is capable of measuring a reference signal relating to the flow of heat from the surface of the second microresonator to the heat sink coupled thermally to the second heat flow sensor. In one embodiment, the first microresonator comprises a piezoelectric substrate having a perimeter, a first face for directly contacting the sample, and a second opposite face isolated from contacting the sample; and further wherein the second microresonator comprises a piezoelectric substrate having a perimeter, a first face, and a second opposite face, the piezoelectric substrates of the first and second microresonators having a resonant frequency and capable of providing a measurement signal based on the resonant frequency. In one embodiment, the sample is a solid sample.
Suitable microresonators for the apparatus of this invention include, but are not limited to, bulk acoustic wave sensors, quartz crystal microbalances, surface acoustic wave sensors, flexural plate wave sensors, and acoustic plate mode sensors. In a preferred embodiment, the first and second microresonators of the apparatus of the present invention are quartz crystal microbalances. In one embodiment, the first microresonator comprises a surface coating with an affinity for at least one component of the sample. In one embodiment, the second microresonator comprises a surface coating with an affinity for at least one component of the sample.
In one embodiment, the first and second heat flow sensors of the apparatus of this invention comprise a thermopile.
Another aspect of the present invention pertains to a mass and heat flow measurement system comprising (i) at least one mass and heat flow measurement sample sensor comprising a microresonator, a heat flow sensor coupled thermally to the microresonator, and a heat sink coupled thermally to the heat flow sensor, wherein the microresonator generates data relating to the changes in mass on a surface of the microresonator arising from contacting the microresonator with a sample; and further wherein the heat flow sensor generates data relating to the changes in flow of heat from the sample to the heat sink from contacting the microresonator with the sample; (ii) at least one mass and heat flow measurement reference sensor comprising a reference microresonator, a reference heat flow sensor coupled thermally to the reference microresonator, and a heat sink coupled thermally to the reference heat flow sensor, wherein the microresonator of the reference sensor is not in contact with the sample; and further wherein the reference microresonator generates data relating to the changes in mass on a surface of the reference microresonator and the reference heat flow sensor generates data relating to the changes in flow of heat from the surface of the reference microresonator to the heat sink coupled thermally to the reference heat flow sensor; and, (iii) a measurement instrument capable of correlating the data from the sample and reference sensors so as to provide measurement of the mass of the sample and the flow of heat from the sample to the heat sink coupled thermally to the heat flow sensor of the sample sensor. In one embodiment of the system of this invention, the microresonator of the sample sensor comprises a piezoelectric substrate having a perimeter, a first face for directly contacting a sample, and a second opposite face isolated from contacting the sample; and further wherein the reference microresonator comprises a piezoelectric substrate having a perimeter, a first face, and a second opposite face, the piezoelectric substrates of the microresonator of the sample sensor and the reference microresonator having a resonant frequency and capable of providing a measurement signal based on the resonant frequency. In one embodiment, the sample is a solid sample.
Suitable microresonators for the sample sensor and suitable reference microresonators for the system of the present invention include, but are not limited to, bulk acoustic wave sensors, quartz crystal microbalances, surface acoustic wave sensors, flexural plate wave sensors, and acoustic plate mode sensors. In a preferred embodiment, the microresonator of the sample sensor and the reference microresonator are quartz crystal microbalances. In one embodiment, the microresonator of the sample sensor comprises a surface coating with an affinity for at least one component of the sample. In one embodiment, the reference microresonator comprises a surface coating with an affinity for at least one component of the sample.
In one embodiment, the heat flow sensor of the sample sensor and the reference heat flow sensor of the system of the present invention comprise a thermopile.
Another aspect of the present invention pertains to a method for measuring the mass of a sample and the flow of heat from the sample to a heat sink, which method comprises the steps of: (i) contacting the sample with a mass and heat flow measurement system, as described herein; (ii) obtaining data from both the sample and reference sensors of the mass and heat flow measurement system of the present invention; and, (iii) determining the mass of the sample and the flow of heat from the sample to the heat sink coupled thermally to the heat flow sensor of the sample sensor. In one embodiment, the sample is a solid sample, and, preferably, the method of this invention measures the enthalpy of sublimation of the solid sample.
Still another aspect of the present invention pertains to a mass and heat flow measurement apparatus comprising (i) a gas sample sensor comprising a first microresonator, a first heat flow sensor, and a heat sink coupled thermally to the first heat flow sensor, wherein the first heat flow sensor is thermally coupled to the first microresonator; and further wherein the first microresonator comprises a coated surface and is capable of measuring the changes in mass when a gas reacts with, is adsorbed, or is desorbed from the coated surface; and the first heat flow sensor is capable of measuring the flow of heat from the coated surface to the heat sink; (ii) a gas reference sensor comprising a second microresonator, a second heat flow sensor coupled thermally to the second microresonator, and a heat sink coupled thermally to the second microresonator; and, (iii) a chamber housing the gas sample and gas reference sensors, wherein the gas reference sensor is isolated from the gas in contact with the first microresonator; and further wherein the second microresonator is capable of measuring a reference signal relating to mass at a surface of the second microresonator, and the second heat flow sensor is capable of measuring a reference signal relating to the flow of heat from the surface of the second microresonator to the heat sink coupled thermally to the second heat flow sensor. In one embodiment, the chamber further comprises a gas input lead for introducing the gas into contact to the coated surface of the first microresonator and a gas output lead for removing the gas from contact to the coated surface of the first microresonator.
Yet another aspect of the present invention pertains to a mass and heat flow measurement system comprising (i) at least one mass and heat flow measurement gas sample sensor comprising a microresonator, a heat flow sensor coupled thermally to the microresonator, and a heat sink coupled thermally to the heat flow sensor, wherein the microresonator comprises a coated surface and generates data relating to the changes in mass on the coated surface of the microresonator arising from contacting the coated surface with a gas; and further wherein the heat flow sensor generates data relating to the changes in flow of heat from the coated surface to the heat sink from contacting the coated surface with the gas; (ii) at least one mass and heat flow measurement gas reference sensor comprising a reference microresonator, a reference heat flow sensor coupled thermally to the reference microresonator, and a heat sink coupled thermally to the reference heat flow sensor, wherein the microresonator of the gas reference sensor is not in contact with the gas in contact with the microresonator of the gas sample sensor; and further wherein the reference microresonator generates data relating to the changes in mass on a surface of the reference microresonator, and the reference heat flow sensor generates data relating to the changes in flow of heat from the surface of the reference microresonator to the heat sink coupled thermally to the reference heat flow sensor; and, (iii) a measurement instrument capable of correlating the data from the gas sample and gas reference sensors so as to provide measurement of the changes in mass on the coated surface of the microresonator of the gas sample sensor and the flow of heat from the coated surface to the heat sink coupled thermally to the heat flow sensor of the gas sample sensor. In one embodiment, the system further comprises a gas input lead for introducing the gas into contact to the coated surface of the microresonator of the gas sample sensor and a gas output lead for removing the gas from contact to the coated surface of the microresonator of the gas sample sensor.
Another aspect of the present invention pertains to a method for measuring the change in mass and flow of heat from a coated surface to a heat sink when a gas reacts with, is adsorbed, or is desorbed from the coated surface, which method comprises the steps of: (i) contacting the gas with a mass and heat flow gas measurement system, as described herein; (ii) obtaining data from both the gas sample and gas reference sensors of the mass and heat flow gas measurement system of this invention; and, (iii) determining the changes in mass and the flow of heat from the coated surface of the microresonator of the gas sample sensor to the heat sink coupled thermally to the heat flow sensor of the gas sample sensor, which changes arise when the gas reacts with, is adsorbed, or is desorbed from the coated surface. In one embodiment of the method, the mass and heat flow gas measurement system further comprises a gas input lead for introducing the gas to the microresonator of the gas sample sensor and a gas output lead for removing the gas from contact to the microresonator of the gas sample sensor. In one embodiment, the method measures the molar heat of adsorption of the gas, and, preferably, the molar heat of adsorption is measured in real time. In one embodiment, the reference microresonator comprises a coated surface. In one embodiment, the coated surface of the reference microresonator comprises the same coating as the coated surface of the microresonator of the gas sample sensor. In one embodiment, the method measures the molar heat of reaction of the gas with the coated surface, and, preferably, the molar heat of reaction is measured in real time. In one embodiment, the method measures the molar heat of desorption of the gas, and, preferably, the molar heat of desorption is measured in real time. In one embodiment, the microresonator of the gas sample sensor and the reference microresonator are selected from the group of microresonators consisting of: bulk acoustic wave sensors, quartz crystal microbalances, surface acoustic wave sensors, flexural plate wave sensors, and acoustic plate mode sensors. In a preferred embodiment, the microresonator of the gas sample sensor and the reference microresonator are quartz crystal microbalances.
Another aspect of the present invention pertains to a heat flow measurement apparatus comprising (i) a liquid sample sensor comprising a first microresonator, a first heat flow sensor, and a heat sink coupled thermally to the first heat flow sensor, wherein the first heat flow sensor is thermally coupled to the first microresonator; and further wherein the first microresonator is capable of measuring a signal relating to mass at a surface of the first microresonator, and the first heat flow sensor is capable Of measuring the flow of heat from a liquid sample disposed on the first microresonator to the heat sink; (ii) a liquid reference sensor comprising a second microresonator, a second heat flow sensor coupled thermally to the second microresonator, and a heat sink coupled thermally to the second microresonator; and, (iii) a chamber housing the liquid sample and liquid reference sensors, wherein the liquid reference sensor is isolated from the liquid sample in contact with the first microresonator; and further wherein the second microresonator is capable of measuring a reference signal relating to mass at a surface of the second microresonator, and the second heat flow sensor is capable of measuring a reference signal relating to the flow of heat from the surface of the second microresonator to the heat sink coupled thermally to the second heat flow sensor. In one embodiment of the heat flow apparatus of this invention, the chamber further comprises a liquid input lead for introducing the liquid sample into contact to the first microresonator. In one embodiment, the first microresonator is capable of measuring the mass of the liquid sample in contact with the first microresonator.
Still another aspect of this invention pertains to a heat flow measurement system comprising (i) at least one heat flow measurement liquid sample sensor comprising a microresonator, a heat flow sensor coupled thermally to the microresonator, and a heat sink coupled thermally to the heat flow sensor, wherein the microresonator is capable of measuring a signal relating to mass at a surface of the microresonator; and further wherein the heat flow sensor generates data relating to the changes in the flow of heat from a liquid sample to the heat sink from contacting the microresonator with the liquid sample; (ii) at least one heat flow measurement liquid reference sensor comprising a reference microresonator, a reference heat flow sensor coupled thermally to the reference microresonator, and a heat sink coupled thermally to the reference heat flow sensor, wherein the microresonator of the reference sensor is not in contact with the liquid sample in contact with the microresonator of the liquid sample sensor; and further wherein the reference microresonator is capable of measuring a reference signal relating to mass at a surface of the reference microresonator, and the reference heat flow sensor generates data relating to the changes in flow of heat from the surface of the reference microresonator to the heat sink coupled thermally to the reference heat flow sensor; and, (iii) a measurement instrument capable of correlating the data from the liquid sample and liquid reference sensors so as to provide measurement of the flow of heat from the liquid sample to the heat sink coupled thermally to the heat flow sensor of the liquid sample sensor. In one embodiment of the heat flow measurement system of the present invention, the system further comprises a liquid input lead for introducing the liquid sample to the microresonator of the liquid sample sensor. In one embodiment, the microresonator of the liquid sample sensor generates data relating to the changes in mass on a surface of the microresonator of the liquid sample sensor, which data arises from contacting the microresonator of the liquid sample sensor with the liquid sample; wherein the reference microresonator generates data relating to the changes in mass on a surface of the reference microresonator; and further wherein the measurement instrument is capable of correlating the data from the liquid sample and liquid reference sensors so as to further provide measurement of the mass of the liquid sample. In one embodiment, the mass of the liquid sample is known.
Another aspect of the present invention pertains to a method for measuring the change in the flow of heat from a liquid sample to a heat sink, which method comprises the steps of: (i) contacting the liquid sample with a heat flow measurement system, as described herein; (ii) obtaining data from both the liquid sample and liquid reference sensors of the heat flow measurement system of this invention; and, (iii) determining the flow of heat from the liquid sample to the heat sink Coupled thermally to the heat flow sensor of the liquid sample sensor. In one embodiment, the heat flow measurement system further comprises a liquid input lead for introducing the liquid sample to the microresonator of the liquid sample sensor. In one embodiment, the microresonator of the liquid sample sensor generates data relating to the changes in mass on a surface of the microresonator of the liquid sample sensor, which data arises from contacting the microresonator of the liquid sample sensor with the liquid sample; wherein the reference microresonator generates data relating to the changes in mass on a surface of the reference microresonator; and further wherein the measurement instrument is capable of correlating the data from the liquid sample and liquid reference sensors so as to further provide measurement of the mass of the liquid sample. In one embodiment, the mass of the liquid sample is known. In one embodiment, the method measures the molar heat of evaporation of the liquid sample. In one embodiment, the microresonator of the liquid sample sensor comprises a coated surface, and, preferably, the reference microresonator also comprises a coated surface, and, most preferably, the coated surface of the reference microresonator comprises the same coating as the coated surface of the microresonator of the liquid sample sensor. In one embodiment, the microresonator of the liquid sample sensor and the reference microresonator are selected from the group of microresonators consisting of: bulk acoustic wave sensors, quartz crystal microbalances, surface acoustic wave sensors, flexural plate wave sensors, and acoustic plate mode sensors. In a preferred embodiment, the microresonator of the liquid sample sensor and the reference microresonator are quartz crystal microbalances.
As one skilled in the art will appreciate, features of one embodiment and aspect of the invention are applicable to other embodiments and aspects of the invention.